The present invention relates to a system for regulating the output of at least one engine of a helicopter.
It is known that the control system of an engine oversees the quantity of fuel which is injected into the combustion chamber of this engine, in the following various operating configurations of the helicopter: startup, flight, engine switch-off. For this purpose, two mechanical controls act on the engine, namely:
a flow rate control which is actuated by the pilot (flow rate lever); and
a control for compensating the regulation of the engine. The latter control, which is coupled to the collective pitch control, is automatic.
The pilot is therefore furnished only with a single direct control, the flow rate lever, which in fact serves only during the switch-on, switch-off phases and during the acceleration of the engine up to the regulated output (flow rate lever in the xe2x80x9cflightxe2x80x9d position). In helicopters equipped with an engine computer, these phases are overseen automatically by the computer: the flow rate lever then becomes unnecessary.
When the flow rate lever is set to the xe2x80x9cflightxe2x80x9d position, the pilot no longer needs to control the flow rate, since the engine is then governed by a regulating system which automatically meters the fuel as a function of the power demanded by the rotor (function of the collective pitch) and keeps the speed of rotation of the free turbine of the engine constant. The control of the regulation acts on the free turbine regulator. Intervening automatically on variation of the collective pitch, it:
compensates partially for the droop of the centrifugal regulator, that is to say preserves a substantially constant NR rotor output regardless of the fuel flow rate, hence regardless of the power demanded; and
affords a very short response time, avoiding pumping during abrupt accelerations and shutdown during abrupt decelerations. For this reason, this control is also referred to as xe2x80x9canticipatorxe2x80x9d, since it antedates the normal reaction of the centrifugal regulator.
To do this, the system for regulating the output of the engine comprises, in a known manner:
at least one metering valve for metering said fuel fed to the engine, as a function of control commands;
a means for measuring an NTL output corresponding to the rotational output of the free turbine of said engine;
a means for measuring an NG output corresponding to the output of the gas generator of said engine;
a device for determining a pre-set value NTLcons corresponding to the pre-set value of the rotational output of the free turbine of the engine. This device is generally an adjustable potentiometer which delivers a constant value. This value can be tuned, if necessary, by the pilot, to obtain values of the NR rotor output which are in accordance with the flight manual; and
computation means for automatically computing the control commands which are applied to said metering valve. Said computation means compute control commands which make it possible to slave, knowing said measured NG output, the output of the gas generator of said engine to a pre-set value NGcons of said output of the gas generator, which depends on said measured NTL output and on said specified pre-set value NTLcons.
This known regulator system has certain drawbacks. In particular, the NR rotational output of the rotor in hovering flight is imposed by the prior choice of the optimal output for level flight and always remains less than the NR output for level flight.
The object of the present invention is to remedy these drawbacks and to enhance this regulating system with the aim of obtaining an output/speed curve which meets particular safety, noise and performance criteria, without modifying the basics of the regulating system.
To this end, according to the invention, said regulating system of the type comprising:
at least one metering valve for metering said fuel fed to the engine, as a function of control commands;
a first means for measuring an NTL output corresponding to the rotational output of the free turbine of said engine;
a second means for measuring an NG output corresponding to the output of the gas generator of said engine;
a device for determining a pre-set value NTLcons corresponding to the pre-set value of the rotational output of the free turbine of the engine; and
computation means for automatically computing the control commands which are applied to said metering valve, said computation means computing control commands making it possible to slave, knowing said measured output, the output of the gas generator of said engine to a pre-set value NGcons of said output of the gas generator, which depends on said measured NTL output and on said specified pre-set value NTLcons,
is noteworthy in that said device comprises at least:
a third means for determining the position of a rudder bar of said helicopter; and
an auxiliary computation means which computes said pre-set value NTLcons as a function of said position of the rudder bar.
Thus, by virtue of allowing for the position Dxcex4 of the rudder bar in the formulation of said pre-set value NTLcons of the rotational output of the free turbine, a particular and advantageous relation between the NR rotor output (the NTL and NR outputs being proportional, on account of the mechanical link existing between the engine and the main gearbox of the helicopter) and the speed Vi of the helicopter is obtained on account in particular of the variation in the position Dxcex4 as a function of the speed Vi of the helicopter, as will be seen in greater detail hereinbelow.
The rotor output regulation thus obtained, which therefore depends on the speed, has numerous advantages and makes it possible in particular:
around the hovering flight, to increase the output to the maximum possible, so as:
to push back the weight limit onwards of which there is no longer any nonsafety zone should the engine fail (broadening of the height/speed chart); and
to increase the effectiveness of the tail rotor (the thrust of the tail rotor increasing with the rotational output);
to limit the output in level flight, so as to reduce the flyover noise; and
not to reduce the PMC (maximum continuous power) level output too much so as not to reduce the maximum speed too much, especially at low altitude.
Moreover, by virtue of the invention, the following advantages are also obtained:
at low weight, the power required for flight is lower, hence the rotor torque is also lower. This results in a smaller anti-torque requirement and hence a lower rudder bar control (Dxcex4), this bringing about a lower NR output since the NR output follows the variations of Dxcex4. Thus, the helicopter will be less noisy, since the output of its rotors will be lower (the output of the tail rotor being proportional to that of the main rotor); and
when climbing at very low speed, when the vertical stabilizer is hardly effective, the rudder bar control (Dxcex4) must increase so as to compensate for the progressive decrease in the density of the air with altitude. This results in an increase in the NR output, thereby improving the efficiency of the rotor (less consumption for equal climb performance). Likewise, while the helicopter is climbing, the pilot increases the overall pitch of the main rotor so as to increase the lift and hence the power and the torque. The anti-torque requirement increases accordingly, thereby also producing an increase in the NR output favorable to performance.
Moreover, the present invention can be implemented in a simple manner and cheaply, simply by adding said auxiliary computation means and said third means for determining the position of the rudder bar. Preferably, this third means is a feedback potentiometer for copying the position (Dxcex4) of the rudder bar which, generally, already exists on a helicopter, to aid the automatic pilot. This facilitates the carrying out of the invention even more.
It will be noted that the present invention is applicable to any type of regulating system, be it electromechanical, hydromechanical or even digital, and is so equally for regulation of proportional type (which, as is known, formulates a pre-set value NGcons of the output of the gas generator of the engine, which is proportional to the discrepancy between said NTL output and said pre-set value NTLcons) and for regulation of integral type (whose pre-set value NGcons alters until the NTL output is equal to the pre-set value NTLcons).
In a preferred embodiment, said auxiliary computation means:
computes the following expression A(Dxcex4):
A(Dxcex4)=aDxcex4+b
in which:
Dxcex4 is the position of the rudder bar; and
a and b are predetermined parameters which take constant values over successive intervals of values of Dxcex4; and
uses this expression A(Dxcex4) to compute said pre-set value NTLcons.
Of course, within the framework of the present invention, this function A can take other forms and in particular all the other possible forms making it possible, for example, to meet particular requirements.
Moreover, advantageously, said device furthermore comprises at least one auxiliary means for determining the value of at least one auxiliary parameter, and said auxiliary computation means uses said value of the auxiliary parameter to compute said pre-set value NTLcons.
In this case, preferably, said auxiliary computation means uses, as auxiliary parameter, at least one of the following parameters:
the altitude of the helicopter;
the speed of the helicopter;
the ambient temperature; and
a pre-set value delivered by an adjustable potentiometer.
The variation of the NR rotor output as a function of the helicopter speed Vi, obtained by virtue of the invention, is especially advantageous in hovering flight, as indicated previously. On the other hand, in the transient phase of flight, two problems may arise, namely:
a safety problem, should the engine fail at takeoff.
Starting from hovering flight, the pilot tilts the rotor forward (cyclic pitching control) so as to accelerate, at the same time as he acts on the rudder bar so as to counter the rotor torque and thus avoid undesirable rotation of the helicopter about its yaw axis (given that at low speed, the vertical stabilizer is ineffective) . As soon as the speed increases, the effectiveness of the stabilizer increases and it is necessary to reduce the action on the rudder bar. The NR output then decreases, since it is slaved to the rudder bar while the speed is still low. If the failure occurs at this moment, the pilot must carry out the tricky manoeuvre of landing with an output close to the minimum of the curve. Since moreover, the output of the rotor decreases very rapidly upon failure (around 50 rpm) whatever the pilot does, the helicopter may rapidly find itself in a very difficult situation through lack of lift; and
a flight quality problem.
With the introduction of the rudder bar position information into the engine regulation, the yawing manoeuvres bring about variations in the NR output. However, these variations are undesirable. Specifically, when the pilot acts on the rudder bar so as to counter a gust of wind or perform a rapid rotation, the NR output must not vary, lest it cause nuisance vertical movements of the helicopter.
To remedy at least these two problems, advantageously, said auxiliary computation means carries out a filtering of the value of the position Dxcex4 of the rudder bar, with a transfer function       1          1      +              τ        ⁢                  xe2x80x83                ⁢        1        ⁢                  xe2x80x83                ⁢        p              ,
xcfx841 being a predetermined parameter, before using this filtered value to compute said pre-set value NTLcons.
Thus, by virtue of this filtering, the effect of the position Dxcex4 of the rudder bar on the variations in the NR rotor output is also filtered, this having the consequence:
of delaying the reduction in the NR output when depressing the foot pedal (Dxcex4) on takeoff, to ensure safety near the ground, this making it possible to remedy the abovementioned safety problem; and
of delaying and attenuating the effectiveness of the position Dxcex4 within the regulation so as to ensure proper behavior during yawing manoeuvres, which are of short duration, thereby making it possible to remedy the abovementioned flight quality problem.
Moreover, in view of the couplings between the position Dxcex4 and the NR output, on the one hand, and the NR and NG outputs, on the other hand, the rudder bar movements (Dxcex4) have an impact on the NR output, and hence on the torque delivered by the engine. The torque variations induced by the controls of the rudder bar might be too abrupt in certain flight configurations and might be detrimental, in particular, to the mechanical and structural assemblies.
To avoid such a problem, advantageously, said auxiliary computation means carries out a filtering of the derivative       ⅆ          ⅆ      t        ⁢      (          D      ⁢              xe2x80x83            ⁢      δ        )  
of the Dxcex4 of the rudder bar, with a transfer function             k      ⁢              xe2x80x83            ⁢      p              1      +              τ        ⁢                  xe2x80x83                ⁢        2        ⁢                  xe2x80x83                ⁢        p              ,
k and xcfx842 being predetermined parameters, before using this filtered value to compute said pre-set value NTLcons.
By virtue of this last filtering, the response of the helicopter to a movement of the rudder bar is improved. This is especially beneficial in respect of flight configurations where the engine is close to a limit of power, torque or NG output, since it avoids limitation overshoots during manoeuvres.
Moreover, in a preferred embodiment, said computation means and said auxiliary computation means are grouped together within a single computer, preferably of the FADEC (xe2x80x9cFull Authority Digital Engine Computerxe2x80x9d) type. The introduction, in the form of electrical signals, of the various parameters and in particular of the position Dxcex4, and the carrying out of the various computation functions are thus rendered especially easy.